Electrical power supply system and a permanent magnet generator for such a system

ABSTRACT

An electrical power supply system including an engine driven permanent magnet generator ( 11 ) operable to produce a variable voltage and high frequency output which is converted by a series train AC/AC conversion circuit ( 14 ) into a fixed voltage and relatively low frequency AC output for supply through a trip switch ( 56 - 58 ) to an external load ( 53 - 55 ). A bypass circuit including a pair of normally-closed parallel connected oppositely biased thyristors ( 66 - 68 ) is connected between the generator output and the output of the AC/AC conversion circuit ( 14 ).

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a National Stage of International ApplicationPCT/GB2004/002062, filed May 13, 2004. Applicant claims foreign prioritybenefits under 35 U.S.C. 119(a)-(d) of the following foreignapplications for patent: United Kingdom Application No. 0311013.7, filedMay 13, 2003, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to an electrical power supply system including asource of AC power which is operable to provide an AC output voltagewhich has at least one phase, a series train AC/AC conversion circuitcomprising first convertor means operable to establish an intermediateDC link by converting the AC output voltage into an intermediate DCvoltage and second convertor means operable to convert the intermediateDC voltage into an AC output voltage having at least one phase forsupply to an external load. Such an electrical power supply system isreferred to in this description as an electrical power supply system ofthe kind referred to hereinbefore. More particularly, although notexclusively, this invention relates to an electrical power supply systemof the kind referred to hereinbefore which may be an electrical powergenerating system and/or a motor starting system. This invention alsorelates to a permanent magnet generator operable to generate a variablevoltage generator output, the generator being provided with rectifyingmeans operable to rectify the variable voltage generator output.

WO 01/56133 discloses an electrical power supply system of the kindreferred to hereinbefore which is an electrical power generating systemand which also includes sensing means operable to monitor theintermediate DC link when the external load is connected across the ACpower output and to provide a feedback signal to the speed control meanswhereby to effect a variation of the variable voltage generator outputand thereby to counter a tendency of the intermediate DC voltage tovary. In order to provide the electrical power generating systemdisclosed by WO 01/56133 with protection from the consequences of ashort circuit when the external load is connected across the AC poweroutput, a trip switch would be provided in the, or each phase of the, ACpower output, the or each trip switch being operable to disconnect theAC power output and the external load in response to short circuitconditions being sensed. The, or each, trip switch would be arranged totrip when current flow through it has been maintained at a certain levelfor a certain time.

Suitable trip switches have a current/time tripping characteristic whichallows a high current flow for a short time, sufficient for starting amotor or switching on lamps, before they are tripped by the high currentor they allow lower levels of current flow for longer periods of timebefore they are tripped. The short circuited load current is liable tobuild up to a high level if the trip switches are arranged to be trippedby a low operating current.

Naturally high frequency self commuted devices such as transistors andother electronic components of the system need to be rated to withstandthe power to which the trip switches may be subjected without tripping.Accordingly, where the second convertor means comprise a transistorarrangement, as is usual, it is desirable for the transistors thereof tobe rated so as to withstand a current several times greater than thecurrent to which they are subjected under normal load operatingconditions as such a higher than normal current is needed to trip thetrip switches with minimal time delay in response to the sensing ofshort circuit conditions. This requirement imposes a significant costpenalty as the cost of a transistor increases significantly with itscurrent rating. Similar trip switches may be employed in a motorstarting system to provide the permanent magnet motor with overcurrentprotection and the foregoing observations having regard to provision oftrip switches in an electrical power generating system such as isdisclosed by WO 01/56133 in order to provide protection from theconsequences of a short circuit when the external load is connectedacross the AC power output apply to the use of such trip switches forovercurrent protection in a motor starting system.

U.S. Pat. No. 6,295,215 issued Sep. 25, 2001 to Faria et al. disclosesan electrical power supply system of the kind referred to hereinbeforewhich is provided with a bypass circuit which connects the or each phaseof the AC output voltage provided by the source of AC power directly toa respective phase of the AC output voltage of the second convertormeans thereby bypassing the series train AC/AC conversion circuit, thebypass circuit including an electronically operable unidirectionalswitch arrangement for the or each phase of the AC output voltageprovided by the source of AC power, the electronically operableunidirectional switch arrangement being operable to enable current flowthrough the bypass circuit as well as through the series train AC/ACconversion circuit when the power supply system is connected to anexternal load, the electrical power supply system also including currentsensing means operable to sense current flow to the external load whenthe power supply system is connected to that load and control meansresponsive to the current sensing means and operable to control theunidirectional switch arrangement and thereby to control current flowthrough the bypass circuit. The electrical power supply system of thekind referred to hereinbefore disclosed in U.S. Pat. No. 6,295,215 iscommonly used in equipment such as uninterrupted (or “uninteruptible”)power supplies (UPSs), motor drives and other applications. In a firstmode of operation of the electrical power supply system of the kindreferred to hereinbefore disclosed by U.S. Pat. No. 6,295,215, theseries train AC/AC conversion circuit may be operated such that itcauses the bypass circuit to predominately transfer real power from thesource of AC power to the external load, that real power being a secondcomponent of power from the source of AC power to the external load, thefirst component of power from the source of AC power to the externalload being transferred by the series train AC/AC conversion circuit. Ina second mode of operation, the series train AC/AC conversion circuittransfers power from the source of AC power to the external load whilethe electronically operable unidirectional switch arrangement of thebypass circuit is open so that no power is transferred from the sourceof AC power to the external load through the bypass circuit.

U.S. Patent Application, Publication No. 2003/0043521, disclosesprovision of a fuse between a power supply and a number of loads inparallel. The fuse is caused to be blown by a control circuit inresponse to sensed fault conditions. This is achieved by connecting thefuse to earth through a bypass circuit which bypasses the loads, thebypass circuit having a lower resistance so that the fuse blows.

An object of this invention is to enable the trip switch or switchesprovided either to protect an electrical power supply system of the kindreferred to hereinbefore which is incorporated in an electrical powergenerating system from the consequences of a short circuit when theexternal load is connected across the AC power output or to provide amotor starting system with overcurrent protection, to be tripped withminimal delay after short circuit or overcurrent conditions are sensedwhilst allowing low cost, low current rated electronic components to beused in the first and second convertors and in any sensing means of theelectrical power supply system of the kind referred to hereinbefore.

According to one aspect of this invention there is provided anelectrical power supply system including a source of AC power which isoperable to provide an AC output voltage which has at least one phase, aseries train AC/AC conversion circuit comprising first convertor meansoperable to establish an intermediate DC link by converting the ACoutput voltage into an intermediate DC voltage and second convertormeans operable to convert the intermediate DC voltage into an AC outputvoltage having at least one phase for supply to an external load, abypass circuit which connects the or each phase of the AC output voltageprovided by said source directly to a respective phase of the AC outputvoltage of the second convertor means thereby bypassing the series trainAC/AC conversion circuit, the bypass circuit including an electronicallyoperable unidirectional switch arrangement for the or each phase of theAC output voltage provided by said source, the electronically operableunidirectional switch arrangement being operable to enable current flowthrough the bypass circuit as well as through the series train AC/ACconversion circuit when the power supply system is connected to anexternal load, current sensing means operable to sense output currentflow from the series train AC/AC conversion circuit when the powersupply system is connected to said load and control means responsive tosaid current sensing means and operable to control said unidirectionalswitch arrangement and thereby to control current flow through saidbypass circuit, wherein a respective trip switch is provided for the oreach phase of the AC output voltage of the second convertor means andthe respective phase of the bypass circuit, the unidirectional switcharrangement being open normally so that normally there is no currentflow through the bypass circuit but the control means being operable toclose the unidirectional switch arrangement in response to the currentsensing means sensing current flow through the series train AC/ACconversion circuit in excess of a predetermined current for apredetermined time so that the flow of current from the series trainAC/AC conversion circuit to the external load through the trip switch orswitches is augmented by the current flow through the bypass circuitwhereby the or each trip switch is caused to trip by the augmentedcurrent flow through it.

According to another aspect of this invention there is provided apermanent magnet generator including two sets of star-connectedmulti-phase stator windings, each of the stator windings of each of thetwo sets being connected between diodes of a respective branch of arespective multi-phase diode rectifier arrangement which has a positiveand a negative output, the positive outputs of each of the rectifierarrangements being for connection to the positive input of the firstconvertor means of an electrical power generating system according tosaid one aspect of this invention, the negative outputs of each of therectifier arrangements being for connection to the negative input ofsaid first convertor means and the common terminal of the two sets ofstar-connected multi-phase windings being for connection to the neutralof electrical power generating system.

Provision in an electrical power supply system of the kind referred tohereinbefore of a bypass circuit in accordance with said one aspect ofthis invention renders a permanent magnet generator arrangementaccording to the other aspect of this invention particularly useful asthe source of AC power of the electrical supply system of the kindreferred to hereinbefore.

In the preferred embodiment of this invention the electronicallyoperable unidirectional switch arrangement is formed of thyristors whichhave a much higher ability to carry high currents than do transistors.Triggering of the thyristors results in the load being supplied directlyfrom the generator by a high current which causes tripping of the tripswitches and thereby protects the components of the AC/AC conversioncircuit. The generator provides a variable high frequency output voltagewhereas the frequency of the output voltage of the AC/AC conversioncircuit is relatively low. This results in the thyristors beingcommutated naturally by the generator output voltage. Short pulses ofcurrent are passed by the bypass circuit, bypassing the AC/AC conversioncircuit when the thyristors are tripped and several such short currentpulses are passed during one half cycle of the output voltage of theAC/AC conversion circuit. The bypass circuit may be arranged to producea large number of current pulses for each half cycle of the output fromthe AC/AC conversion circuit and that leads to a high quality outputvoltage and current from the system. This may be achieved by providing alarge number of bypasses, each with its own unidirectional switcharrangement. For example, each phase of the generator output voltage maybe connected to each phase of the output of the AC/AC conversion circuitvia its own bypass including its own unidirectional switch arrangement.Such an arrangement is not only suitable for providing short circuitprotection but it is also suitable for providing current overloadprotection in a motor starting system.

An electric power supply system according to one embodiment of thepresent invention comprises a source of AC power which is operable toprovide an AC output voltage, a series train AC/AC conversion circuitincluding a first converter operable to establish an intermediate DClink, a bypass circuit connecting the AC output voltage provided by thesource to a second converter for bypassing the series train AC/ACconversion circuit, a current sensor operable to sense output currentflow from the series train AC/AC conversion circuit and a control unitresponsive to the current sensor and operable to control aunidirectional switch arrangement and thereby to control current flowthrough the bypass circuit. Further, a trip switch is provided for an ACoutput voltage of the second converter and the bypass circuit whereinthe unidirectional switch arrangement is normally open so that normallythere is no current flow through the bypass circuit, the control unitbeing operable to close the unidirectional switch arrangement.

Several forms of electrical power supply systems in which this inventionis embodied will be described now by way of example with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of one form of electrical powergenerating system, according to a typical embodiment of the presentinvention.

FIG. 2 is a diagrammatic representation of the AC/AC conversion circuitof the electrical power generating system shown in FIG. 1.

FIG. 3 is a graph of current against time and is an illustration of thetripping characteristics of each of the trip switches of the systemillustrated in FIG. 1.

FIG. 4 is a diagrammatic representation of another form of electricalpower generating system, according to another embodiment of the presentinvention.

FIG. 5 is a diagrammatic representation of a further form of electricalpower generating system, according to another embodiment of the presentinvention.

FIG. 6 is a diagrammatic representation of yet another form ofelectrical power generating system, according to another embodiment ofthe present invention.

FIG. 7 is a diagrammatic representation of another form of electricalpower generating system, according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

FIG. 1 shows components of an electrical power generating set indiagrammatic form. The generating set includes a prime mover 10, such asa diesel engine, mechanically coupled with a rotor of an electric powergenerator whereby to rotate the rotor relative to a stator of thegenerator 11 and thereby to generate a three phase variable frequencyand voltage AC output from the generator 11. The preferred form ofelectric power generator 11 is a permanent magnet generator.

The prime mover 10 has an electronic speed sensor 12, a speed controller13 and an electronically controlled fuel injection system. The speedsensor 12 produces an output signal Ssp which is indicative of thesensed prime mover speed. The speed controller 13 has four input ports.One of those input ports receives a minimum speed reference signalSrmis. Another of those input ports receives the actual speed signal Sspwhich is emitted by the speed sensor 12. A third of the input ports ofthe speed controller 13 receives a maximum speed reference signal Srmaxand the fourth input port receives a speed correction signal Srs whichis generated by operation of the generating set as is described below.The speed controller 13 controls operation of the fuel injection systemof the prime mover 10 in response to the four input signals Ssp, Srs,Srmis and Srmax in order to control the prime mover 10 and to maintainthe speed of the prime mover 10 between the minimum and maximum speedsand at a level which is related to the speed correction signal Srs.

The three phase variable frequency and voltage output R, S, T of thegenerator 11 is connected to a series train AC/AC conversion circuit 14.FIG. 2 shows that the series train AC/AC conversion circuit 14 comprisesa first converter 15, which is a twin rectifier circuit and a twinvoltage booster circuit 16, a brake controller 30 and a second converter17 which is a three phase DC/AC inverter. The twin rectifier circuitcomprises a common cathode three pulse rectifier 18 and a common anodethree pulse rectifier 19. The output R, S, T of the generator 11 isconnected in parallel to the anodes of the common cathode three pulserectifier 18 and to the cathodes of the common anode three pulserectifier 19 and is rectified by each of the two rectifiers 18 and 19.Each of the rectifiers 18 and 19 has an output terminal. The outputterminal that is connected to the cathodes of the rectifier 18 ispositive and the output terminal that is connected to the anodes of therectifier 19 is negative. The generator 11 has a neutral terminal N.

Each booster circuit part of the twin voltage booster circuit 16 isconnected between a respective one of the output terminals of therectifiers 18 and 19 and the neutral terminal N, conveniently via afilter which is not shown. Each booster circuit part includes aninductor 21, 22 which is connected in series between the output of therespective rectifier 18, 19 and one side of a respective current sensor23, 24. The other side of the current sensor 23 is connected to thecollector of a transistor 25 and to the anode of a diode 26.

The other side of the current sensor 24 is connected to the emitter of atransistor 27. The emitter of the transistor 25 and the collector of thetransistor 27 are both connected to the neutral terminal N. The otherside of the current sensor 24 is also connected to the cathode of adiode 28. The cathode of the diode 26 is connected to one side of acapacitor 29. The anode of the diode 28 is connected to one side ofanother capacitor 31. The other side of each of the capacitors 29 and 31is connected to the neutral terminal.

The voltage across the capacitor 29 is boosted by the combined effect ofthe inductor 21 and the switching action of the parallel connectedtransistor 25 and the series connected diode 26. The voltage across thecapacitor 31 is boosted by the combined effect of the inductor 22 andthe switching action of the parallel connected transistor 27 and theseries connected diode 28. The output of the twin booster circuit 16 isthe voltage across the capacitors 29 and 31 and the sum of thosevoltages is maintained constant and forms the DC link voltage.

The brake controller 30 is connected across the DC link voltage. Thebrake controller 30 includes a resistor Rh in series with a transistorTh, the resistor Rh being connected to the collector of the transistorTh. A reverse biased diode Dh is connected in parallel with the resistorRh. The base of the transistor Th is connected to a brake voltagecontroller which is not shown.

A voltage sensor 32 is connected across the capacitor 29 and thereby isoperable to monitor the voltage Va across that capacitor 29. Further thevoltage sensor 32 is also connected across the capacitor 31 and therebyis operable to monitor the voltage Vb across that capacitor 31. Hencethe voltage sensor 32 is operable to monitor the DC link voltage. Thevoltage sensor 32 emits two output signals SVa and SVb which arerespectively an indication of the voltage across the capacitor 29 andthe capacitor 31.

The output of each current sensor 23, 24 is connected to an input of arespective current controller 33, 34 which provides a pulse widthmodulated drive to the base of the respective booster transistor 25,27which sources a controlled current to maintain the voltage Va, Vb acrossthe respective capacitor 29, 31 constant as regulated by a respectivevoltage controller 35, 36.

Each of the voltage controllers 35 and 36 has four input ports andreceives at one of those input ports the respective one of the outputsignals SVa and SVb emitted by the voltage sensor 32. Each voltagecontroller 35, 36 receives a respective reference signal SVar, SVbr atanother of its input ports. The output signal Ssp from the speed sensor12 is fed to a third of the input ports of each voltage controller 35,36 and a reference signal St is fed to the fourth input port. Thereference St is the maximum permitted current. It translates to alimiting speed for a given steady state power demand since current isvaried by varying the speed of the prime mover 10. The reference St ischosen as being the current that flows when the prime mover 10 isoperated at a certain working speed which is between the selectedmaximum and minimum engine speeds Srmax and Srmis and when a load isconnected across the intermediate DC voltage. Each voltage controller35,36 has one output which emits the respective current demand signalSImxa, SImxb which is fed to an input of the respective currentcontroller 33,34.

The second converter 17 has a positive input which is connected to thecathode of the diode 26 and to one side of the capacitor 29 and anegative input which is connected through a third current sensor 37 tothe anode of the diode 28 and to one side of the capacitor 31. Thesecond converter 17 has a power output terminal U, V, W for each phasewhich is for connection to an external load. The output of each phaseU,V,W is connected through a respective filter circuit comprising arespective series connected inductor 38, 39, 41 and capacitor 42, 43, 44to the neutral terminal.

The third current sensor 37 measures current flow between the twinbooster circuit 16 and the second converter 17. The current sensor 37emits a signal Sldb which is fed to one of three inputs of a speedcorrection circuit 45. Another input of the speed correction circuit 45receives a reference signal SrLC and the output of the speed correctioncircuit 45 is the signal Srs that is fed to the speed controller 13.

The AC power output for each phase U, V, W of the three-phase output ofthe second converter 17 is produced by operation as bistable switchingmeans of a respective one of three pairs of transistors 46 and 47; 48and 49; 51 and 52 which are connected in parallel with one another andwith the pair of capacitors 29 and 31. The series connection between thetransistors of each pair 46 and 47, 48 and 49, 51 and 52 is connected tothe respective power output terminal U, V, W through the respectiveinductor 38, 39, 41. These power output terminals U, V, W are the poweroutput terminals of the AC/AC conversion circuit 14 shown in FIG. 1.

When each power output U, V, W of the AC/AC conversion circuit 14 isconnected to an external load 53, 54, 55, a trip switch 56, 57, 58 isconnected between the respective power output U,V,W and the neutral lineN of the system through the respective load 53, 54, 55, each trip switch56-58 having one pole connected to the respective power output U, V, Wand another pole connected to the respective load 53-55. Another currentsensor 59, 61, 62 for each phase (see FIG. 2) is connected between therespective power output U, V, W and the respective trip switch 56, 57,58 on either one side (as shown) or the other of the respective inductor38, 39, 41 to measure the current flow through that inductor 38, 39, 41.A voltage sensor 63 senses the output voltage between each line that isconnected to each power output terminal U, V, W and the neutral N. Eachtrip switch 56, 57, 58 is arranged to trip and disconnect the respectiveload 53, 54, 55 when current flow in that line is at a certain level fora certain time. The curve shown in FIG. 3 is a plot of the currentvalues and elapsed time at which a trip switch 56-58 trips.

The reference levels for the required values of output frequency andvoltage produced by normal operation of the generating set are providedby an amplitude and frequency correction circuit 64 (see FIG. 2) whichhas seven inputs and three outputs. It receives the output signals SVaand SVb from the voltage sensor 32 that indicate the voltages across thecapacitors 29 and 31 which together comprise the DC link voltage, atrespective ones of the inputs. At the remaining five inputs it receivesrespective reference signals SRIF, SRRF, Sru1, Srv1 and Srw1. Threeoutputs Sru2, Srv2 and Srw2 from the voltage, amplitude and frequencycorrection circuit 64 are fed to respective ones of three inputs of aninverter controller 65. The inverter controller 65 has another inputwhich is connected to the neutral line N. It also receives three inputsignals Svu, Svv and Svw from the voltage sensor 63 and three inputsignals SIu, SIv, SIw from the respective current sensor 59, 61, 62 thatmeasures the current flow through the respective inductor 38, 39, 41.The inverter controller 65 controls the switching operation of thetransistors 46, 47, 48, 49, 51 and 52 of the three pairs of transistorsof the inverter 17 by emitting a respective pulse-width modulationsignal from a respective output Tau, Thu, Tav, Tbv, Taw, Tbw which isconnected to the base of the respective transistor 46, 47, 48, 49, 51,52. As a result, the AC output U, V, W of each phase has a substantiallyfixed frequency and voltage and it is that substantially fixed frequencyand voltage AC output which normally is supplied to the respective load53-55 through the respective trip switch 56-58.

Each phase of the variable frequency and voltage AC output of thegenerator 11 is connected through a respective switch arrangement 66,67, 68 to a respective one of the three power outputs U, V, W of theAC/AC conversion circuit 14. Each switch arrangement 66, 67, 68comprises a parallel connected pair of electronically operatedunidirectional switches Thr1 and Thr2, Ths1 and Ths2, and Tht1 and Tht2.Each electronically operated unidirectional switch Thr1, Thr2, Ths1,Ths2, Tht1, Tht2 is a thyristor. The switches Thr1 and Thr2, Ths1 andThs2, Tht1 and Tht2 of each pair are biased in the opposite directionsso that, when turned on, one of them Thr1, Ths1, Tht1 transmits therespective phase of the three phase output of the generator 11 when thatphase is positive whilst the other switch of the parallel connectedpair, Thr2, Ths2, Tht2 transmits the respective phase when that phase isnegative.

The unidirectional switches Thr1-Tht2 are connected to the control meansof the generating set which is adapted to turn each of them on and off.In one embodiment of this invention, each unidirectional switchThr1-Tht2 is connected to a respective output of the inverter controller65 which receives the input signals SIu, SIv and SIw from the respectivecurrent sensors 59, 61 and 62 which measure current flow from the AC/ACconversion circuit 14 to the respective load 53-55, the invertercontroller 65 being adapted to emit a firing signal which triggers eachof the unidirectional switches Thr1-Tht2 from its off state to on.

During normal operation of the system, the unidirectional switchesThr1-Tht2 are in their turned off state so that there is no current flowthrough them to the power output U, V, W. In the event that the currentsensors 59, 61 and 62 sense that current has flown through any one ofthe outputs U, V, W at a certain level for a certain time commensuratewith a short circuit or with overcurrent conditions in a motor startingsystem and that there is a need to trip the respective trip switches56-58, the inverter controller 65 effects turning on of theunidirectional switches Thr1-Tht2 and speeding up of the generator 11 byway of a suitable connection between the inverter controller 65 and thethird input of the speed correction circuit 45. This leads to anincrease in the generator output. That output is fed directly to thepower output U, V, W through the respective switches Thr1-Tht2 so that asignificantly higher current is fed to those outputs bypassing the AC/ACconversion circuit 14 and instantly effecting tripping of the tripswitches 56, 57 and 58. The frequency of the AC output of the generator11 is significantly higher than the frequency of the 3-phase output U,V, W of the AC/AC conversion circuit 14, say of the order of twice ashigh. Accordingly, the generator output voltage changes from positive tonegative during a half cycle of the output voltage at the terminals U,V, W. As a result, the unidirectional switches Thr1-Tht2 will be turnedoff naturally when the generator output voltage falls below the outputvoltage of the AC/AC conversion circuit 14 and when the current throughthe respective unidirectional switch Thr1-Tht2 falls to zero.

However, in the preferred embodiment of this invention that isillustrated in FIGS. 1-3, provision is made for synchronising triggeringof the unidirectional switches Thr1-Tht2 from their off states to onwith the substantially fixed frequency of the output of the AC/ACconversion circuit 14 and with the variable frequency of the output ofthe generator 11, the latter frequency being of the order of twice thatof the former.

Accordingly, a further current sensor 69, 71, 72 for each phase (seeFIG. 1) is connected between the respective switch arrangement 66, 67,68 and the respective power output U,V,W of the A/C conversion circuit14. Each current sensor 69, 71, 72 produces an actual current signalIui-act, Ivi-act, Iwi-act which is indicative of current flow in therespective phase of the bypass circuit through the switchingarrangements 66-68.

FIG. 1 includes a diagrammatic illustration of a system for controllingswitching on of the thyristors Thr1, Thr2, Ths1, Ths2, Tht1, Tht2 of theswitch arrangements 66-68 to provide the synchronisation referred toabove. This control system includes an integrated voltage sensor 73which senses the output voltage between each line that is connected toeach power output terminal U, V, W and the neutral N. The sensor 73shown in FIG. 1 preferably is the same sensor as the voltage sensor 63shown in FIG. 2 but it could be another sensor. Either way, the sensor73 is operable to emit an actual voltage output signal Vui-act, Vvi-act,Vwi-act for each phase and an rmns output voltage signal Vurms-act,Vvrms-act, Vwrms-act for each phase.

There are three rms voltage regulators 74, 75, 76, one for each phase.Each rms voltage sensor 74, 75, 76 receives the respective rms outputvoltage signal Vurms-act, Vvrms-act, Vwrms-act at one input from theintegrated voltage sensor 73 and a reference rms voltage signal Vrms-refat another input. Each rms voltage regulator 74, 75, 76 produces arespective reference current signal Iui-act, Ivi-act, Iwi-act at itsoutput. Normally each actual rms output voltage signal Vurms-act,Vvrms-act, Vwrms-act is greater in magnitude than the reference rmsvoltage signal Vrms-ref in which case the respective output referencecurrent signal Iui-ref, Ivi-ref, Iwi-ref is zero. However, when currentflow in the respective power output line U, V, W of the AC/AC conversioncircuit rises to a certain level for a certain time commensurate with ashort circuit or with overcurrent conditions in a permanent magnet motordriving system so that there will be a need to cause the respective tripswitch 56, 57, 58 to trip and disconnect the respective load, therespective actual rms output voltage signal Vurms-act, Vvrms-act,Vwrms-act will be reduced to below the reference rms voltage signalVrms-ref in which case the respective output reference current signalIui-ref, Ivi-ref, Iwi-ref will be a sinusoidal signal with an amplitudewhich is greater than zero.

There are three current regulators 77, 78, 79, one for each phase. Eachcurrent regulator or 77, 78, 79 has three inputs and one output. Eachcurrent regulator 77, 78, 79 receives the respective actual voltageoutput signal Vui-act, Vvi-act, Vwi-act at one input from the integratedvoltage sensor 73, the respective reference current signal Iui-ref,Ivi-ref, Iwi-ref at another input from the respective rms voltageregulator 74, 75, 76 and the actual current signal Iui-act, Ivi-act,Iwi-act from the respective current sensor 69, 71, 72. Each currentregulator 77, 78, 79 produces a respective reference signal Aru, Asv,Atw at its output when the amplitude of the respective sinusoidalreference current signal Iui-ref, Ivi-ref, Iwi-ref is greater than zero.The respective reference signal Aru, Asv, Atw that is emitted from eachcurrent regulator 77,78,79 when the respective trip switch 56, 57, 58has been tripped, is known as an angle signal. The respective referencesignal Aru, Asv, Atw is positive when the respective actual voltageoutput signal Vui-act, Vvi-act, Vwi-act is positive and is negative whenthe respective actual voltage output signal Vui-act, Vvi-act, Vwi-act isnegative.

Each reference signal Aru, Asv, Atw is fed to a thyristor firingcontroller 81. A synchronisation unit 82 has four inputs and one output.Each of the four inputs of the synchronisation unit 82 is connected to arespective one of the power outputs R, S, T and to the neutral N of thegenerator 11. The output of the synchronisation unit 82 is indicative ofthe instantaneous electrical angle of each phase R, S, T of the outputvoltage and is connected to a fourth input of the thyristor firingcontroller 81.

The thyristor firing controller 81 transforms each angle signal Aru,Asv, Atw it receives into a respective thyristor firing signal andcontrols those thyristor firing signals in relation to the electricalangle of the respective phase R, S, T of the generator output voltage.When each angle signal Aru, Asv, Atw is positive, the respectivethyristor firing signal is directed to trigger the respective thyristorThr1, Ths1, Tht1 of the respective phase R, S, T of the generatoroutput. When each angle signal Aru, Asv, Atw is negative, the respectivethyristor firing signal is directed to trigger the other respectivethyristor Thr2, Ths2, Tht2 of the respective phase R, S, T.

Each angle signal Aru, Asv, Atw determines the electrical angularposition in the respective cycle at which the respective thyristorThr1-Tht2 is to be triggered from its off-state to on. It will beappreciated that logic devices could be used to determine to whichthyristor Thr1-Tht2 each thyristor firing signal is directed to triggerthat thyristor Thr1-Tht2 from its off state to on when the sinusoidaloutput voltage Vui-act, Vvi-act, Vwi-act is positive and when it isnegative.

Hence, whenever the respective phase of the power output of the AC/ACconversion circuit 14 is positive, as it will be for about 10 msecs.,and the amplitude of the respective output reference current signalIui-ref, Ivi-ref, Iwi-ref is greater than zero, the thyristor Thr1,Ths1, Tht1 that transmits the generator output current when it ispositive will be triggered repeatedly so that a plurality of positivepulses of generator output current will be transmitted through therespective switch arrangement 66-68 to the output U, V, W for theduration of a positive half cycle of the power output of the AC/ACconversion circuit 14. Further, provision can be made to synchronise thetiming of triggering of the thyristors Thr1-Tht2 with the sinusoidalform of the generator output current and with that of the output of theAC/AC conversion circuit 14 so as to maximise the number of pulses ofgenerator output current that are fed through the respective bypasscircuit switch arrangement 66-68 during one half cycle of the outputcurrent of the AC/AC conversion circuit 14.

It is not essential for an electrical power generating system whichincludes an asynchronous bypass circuit with high current unidirectionalswitches such as thyristors for short circuit or current overloadprotection as has been described above with reference to FIGS. 1 to 3,to include the neutral line N as shown in FIG. 1. The principle ofoperation of the bypass in such a system would be similar to that thathas been described with reference to FIGS. 1 to 3.

FIG. 4 shows a system which differs from that described above withreference to FIGS. 1 to 3 in the detailed arrangement of the generator,the rectifiers and the electronically operated unidirectional switches.The diagrammatic illustration of the system for controlling switching ofthe thyristors is omitted for convenience. The generator PMG shown inFIG. 4 is a permanent magnet generator which has two three phasevariable frequency and voltage outputs PMG1 and PMG2. Each of the twothree phase variable frequency and voltage outputs PMG1 and PMG2 isrectified by a respective one of two full wave bridge rectifiers Re1 andRe2. Each bridge rectifier Re1, Re2 has a connection to a neutralterminal and has a power output terminal which, with respect to theneutral, is positive in the case of the bridge rectifier Re1 andnegative in the case of the bridge rectifier Re2. The neutral N of theAC power output of the generating set shown in FIG. 4 is connected tothe common neutral terminal of the two bridge rectifiers Re1 and Re2instead of being connected to the neutral terminal of the generator 11as was described above and illustrated in FIGS. 1 and 2.

Each phase of the three phase AC output from the three phase generatorwindings PMG1 is connected through a respective electronically operatedunidirectional switch Tyup, Tyvp, Tywp which is a thyristor, to arespective one of the three power outputs U, V and W of the system.Likewise each phase of the three phase AC output from the three phasegenerator windings PMG2 is connected through a respective electronicallyoperated unidirectional switch Tyun, Tyvn, Tywn, which again is athyristor, to a respective one of the three power outputs U, V and W ofthe system. The three switches Tyup, Tyvp, Tywp are biassed to transmitthe respective phase of the three phase output from the generatorwindings PMG1 when that phase is positive and the other three switchesTyun, Tyvn and Tywn are oppositely biassed so as to transmit therespective phase of the three phase output from the generator windingsPMG2 when that phase is negative. Again, the unidirectional switchesTyup-Tywn are connected to the control means of the system which isadapted to trigger each of them from their off state to on by anappropriate thyristor firing signal. The system described above withreference to and as illustrated in FIG. 4 otherwise operates in asimilar manner to that described above with reference to FIGS. 1 to 3.

FIG. 5 shows another modification of the system described above withreference to and as illustrated in FIGS. 1 and 2 in which the threephase generator 11 and the rectifiers 18 and 19 of the system shown inFIGS. 1 and 2 are replaced by a different topology of permanent magnetgenerator and rectifiers Re1, Re2, Re3 and Re4.

The permanent magnet generator PMG shown in FIG. 5 has a star connecteddouble three phase stator winding PMG1 and PMG2. The common terminal ofthe star connected double three phase stator windings PMG1 and PMG2 isconnected to a neutral line N. Each of the other ends of the doublethree phase stator windings PMG1 and PMG2 is connected between arespective anode of a common cathode three pulse rectifier Re1, Re2 anda respective cathode of a common anode three pulse rectifier Re3, Re4.The common cathode of each of the three pulse rectifiers Re1 and Re2 isconnected through a respective one of a pair of coupled inductors Lra tothe inductor Lba of the positive line of the booster circuits of thesystem. Each of the common anodes of the three pulse rectifiers Re3 andRe4 is connected through a respective one of two coupled inductors Lrbto the inductor Lbb of the negative line of the booster circuits of thesystem. The coupled inductors Lra assure parallel operation of thecommon cathode three pulse rectifiers Re1 and Re2. Likewise the coupledinductors Lrb assure parallel operation of the common anode three pulserectifiers Re3 and Re4. Thus a difference between the DC outputrectified voltages from the respective pair of common cathode threepulse rectifiers Re1 and Re2 is reduced by the coupled inductors Lra anda difference between the DC output rectified voltages from therespective pair of common anode three pulse rectifiers Re3 and Re4 isreduced by the respective coupled inductors Lrb. As a result, the DCoutput connected to the inductor Lba, Lbb of the respective positive ornegative line supplying the booster circuits of the system has thecharacteristics of the output of a six pulse rectifier.

The three windings PMG1 of the star connected double three phase statorwindings of the permanent magnet generator PMG, that are connectedbetween the common cathode three pulse rectifier Re1 and the commonanode three pulse rectifier Re3 are also each connected through arespective one of the electronically operated switch arrangements Tu,Tv, Tw to a respective one of the three power outputs U, V, W of thesystem. Operation of the system is otherwise as described above withreference to and as illustrated in FIGS. 1 to 3.

FIG. 6 shows a system that is similar to the system described above withreference to and as illustrated in FIG. 5. The difference is that eachof the three windings of the other group of star connected three phasestator windings PMG2 of the permanent magnet generator PMG that areconnected between the common cathode three pulse rectifier Re2 and thecommon anode three pulse rectifier Re4 are also connected to arespective one of the three power output terminals U, V, W through arespective switch Tu2, Tv2 and Tw2. Each of the switches Tu2, Tv2 andTw2 comprises a parallel connected pair of oppositely biasedunidirectional switches, Tyup2 and Tyun2; Tyvp2 and Tyvn2; and Tywp2 andTywn2, those switches being thyristors. The operation of the systemshown in FIG. 6 will be understood from the foregoing description.

FIG. 7 shows a modified form of asynchronous bypass circuit for use inthe electrical power generating system described above with reference toFIGS. 1 to 3 which renders it particularly suitable for use as a motorstarting system. Parts of the system shown in FIG. 7 that correspond tolike parts of the system shown in FIG. 1 are identified by the samereference character. By this modification, each phase R, S, T of thevariable voltage and high frequency output of the generator 11 isconnected to each phase U, V, W of the relatively low fixed frequencyoutput of the AC/AC conversion circuit 14 through a respective switcharrangement. More specifically, in addition to being connected to theoutput phase U by the switch arrangement 66, the generator output phaseR is connected to the output phase V by a switch arrangement 83 and tothe output phase W by a switch arrangement 84. Also, in addition tobeing connected to the output phase V by the switch arrangement 67, thegenerator output phase S is connected to the output phase U by a switcharrangement 85 and to the output phase W by a switch arrangement 86.Further, in addition to being connected to the output phase W by theswitch arrangement 68, the generator output phase T is connected to theoutput phase U by a switch arrangement 87 and to the output phase V by aswitch arrangement 88.

In the event that the current sensors 59, 61 and 62 sense that currenthas flown through any one of the outputs U, V, W at a certain level fora certain time commensurate with a short circuit or with overcurrentconditions in a motor starting system and that there is a need to tripthe respective trip switches 66,83,84 or 67,85,86 or 68,87,88, a controlsystem constructed according to the principles described above withreference to FIG. 1 will control triggering of those groups of threeswitches so that three times as many pulses of current are fed one afteranother through each bypass circuit to the respective phase U, V, W ofthe output of the AC/AC conversion circuit 14 during each half cycle ofthe latter than is the case with the arrangement described above withreference to FIG. 1, with the result that high quality voltage andcurrent suitable for starting a motor will be supplied.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

1. An electric power supply system comprising: a source of AC powerwhich is operable to provide an AC output voltage; a series train AC/ACconversion circuit comprising a first converter operable to establish anintermediate DC link by converting the AC output voltage into anintermediate DC voltage and a second converter operable to convert theintermediate DC voltage into an AC output voltage for supply to anexternal load; a bypass circuit which connects the AC output voltageprovided by said source directly to the AC output voltage of the secondconverter thereby bypassing the series train AC/AC conversion circuit,the bypass circuit including an electronically operable unidirectionalswitch arrangement for the AC output voltage provided by said source,the electronically operable unidirectional switch arrangement beingoperable to enable current flow through the bypass circuit as well asthrough the series train AC/AC conversion circuit when the power supplysystem is connected to an external load; a current sensor operable tosense output current flow from the series train AC/AC conversion circuitwhen the power supply system is connected to said load; and a controlunit responsive to said current sensor and operable to control saidunidirectional switch arrangement and thereby to control current flowthrough said bypass circuit, wherein a trip switch is provided for theAC output voltage of said second converter and said bypass circuit, saidunidirectional switch arrangement being open normally so that normallythere is no current flow through said bypass circuit and said controlunit being operable to close said unidirectional switch arrangement inresponse to said current sensor sensing current flow through said seriestrain AC/AC conversion circuit in excess of a predetermined current fora predetermined time so that the flow of current from said series trainAC/AC conversion circuit to said external load through said trip switchis augmented by current flow through said bypass circuit whereby theswitch is caused to trip by the augmented current flow through it.
 2. Anelectrical power supply system according to claim 1, wherein said sourceof AC power includes a generator operable to generate said AC outputvoltage.
 3. An electrical power supply system according to claim 2,wherein said generator is operable to generate a variable voltagegenerator output as said AC output voltage of said source of AC power,said source of AC power also including a variable speed prime moverdrivingly coupled with the generator and a speed controller operable tocontrol the speed of the prime mover and said electrical power supplysystem further including a sensor operable to monitor the intermediateDC link when the external load is connected across the AC power outputand to provide a feedback signal to the speed controller whereby toeffect a variation of the variable voltage generator output and therebyto counter a tendency of the intermediate DC voltage to vary.
 4. Anelectrical power supply system according to claim 1, which is a motorstarting system, wherein the AC output voltage of the second converteris connected to a stator winding of the permanent magnet motor.
 5. Anelectrical power supply system according to claim 1, wherein theelectronically operable unidirectional switch of the arrangement is athyristor.
 6. An electrical power supply system according to claim 3wherein the variable voltage generator output is a multiphase output. 7.An electrical power supply system according to claim 6, wherein at leastone phase of the variable voltage generator output is connected to apositive input of the first converter and another phase of the variablevoltage generator output is connected to a negative input of the firstconverter.
 8. An electrical power supply system according to claim 5,wherein the bypass circuit includes a pair of oppositely biassedthyristors which are connected in parallel to the respective phase ofthe AC output voltage of the second converter.
 9. An electrical powersupply system according to claim 8, wherein each phase of the generatoroutput is connected to the respective phase of the AC output voltage ofthe second converter by the bypass circuit through a respective pair ofoppositely biassed thyristors.
 10. An electrical power supply systemaccording to claim 8, wherein each phase of the multiphase variablevoltage generator output is connected to the respective phase of the ACoutput voltage of the second converter by the bypass circuit through arespective thyristor.
 11. An electrical power supply system according toclaim 10, wherein each thyristor by which said phases of the variablevoltage generator output are connected to the respective phase of theoutput of said second converter are oppositely biassed to each thyristorby which said other phases are connected to the respective phase of saidoutput of the second converter.
 12. An electrical power supply systemaccording to claim 5, wherein each thyristor is turned off during normaloperation of the system and is turned on by operation of said controlunit when current flow through said series train AC/AC conversioncircuit in excess of a predetermined current for a predetermined time issensed.
 13. An electrical power supply system according to claim 12,wherein said control unit is operable to activate said speed controllerto speed up the prime mover when current flow through said series trainAC/AC conversion circuit in excess of a predetermined current for apredetermined time is sensed.
 14. An electrical power supply systemaccording to claim 7, wherein said first converter includes one fullwave rectifier to which one phase of the variable voltage generatoroutput is connected and another full wave rectifier to which anotherphase of the variable voltage generator output is connected, each of thetwo full wave rectifiers having a terminal which is connected toneutral.
 15. An electrical power supply system according to claim 2,wherein the permanent magnet generator includes two sets ofstar-connected multi-phase stator windings, each of the stator windingsof each of the two sets being connected between diodes of a respectivebranch of a respective multi-phase diode rectifier arrangement which hasa positive and a negative output, the positive outputs of the rectifierarrangements being connected to the positive input of the firstconverter, the negative outputs of the rectifier arrangements beingconnected to the negative input of said first converter and the commonterminal of the two sets of star-connected multi-phase windings beingconnected to neutral.
 16. An electrical power supply system according toclaim 14, wherein each of the positive and negative outputs of therectifier arrangements is connected to the respective input of the firstconverter through a respective multi-coupled winding inductancearrangement.
 17. An electrical power supply system according to claim 1,wherein said control unit is operable to synchronize closing of saidunidirectional switch arrangement for each phase of the AC outputvoltage provided by said source with the frequency of the respective ACoutput voltage provided by said source and with the frequency of therespective phase of the AC output voltage of the second converter. 18.An electrical power supply system according to claim 17, in which thefrequency of the AC output voltage provided by said source is higherthan the frequency of the AC output voltage of the second converter andsaid control unit is adapted to control closure of said unidirectionalswitch arrangement for each phase such that, during one positive halfcycle of the respective phase of the AC output voltage of said secondconverter, a series of positive current pulses are fed from said sourceto the respective phase of the AC output of said second converter viathe respective unidirectional switch arrangement.
 19. An electricalpower supply system according to claim 18, wherein said control unit isoperable to monitor the rms value of each phase of the AC output voltageof said second converter and to trigger switching on of theunidirectional switch arrangement of the respective bypass circuit forthat phase when that rms voltage value is less than a certain referencevalue.
 20. An electrical power supply system according to claim 17,wherein each phase of the AC output voltage provided by said source isconnected to each phase of the AC output voltage of said secondconverter through a respective bypass circuit having its ownunidirectional switch arrangement.
 21. An electrical power supply systemaccording to claim 20, wherein said control unit is operable to controltriggering of each unidirectional switch arrangement so that pulses ofcurrent are fed from said source to the AC output of said secondconverter through each unidirectional switch arrangement, one afteranother.
 22. An electrical power supply system according to claim 1,said source of AC power comprising a permanent magnet generatorincluding two sets of star-connected multi-phase stator windings, eachof the stator windings of each of the two sets being connected betweendiodes of a respective branch of a respective multi-phase dioderectifier arrangement which has a positive and a negative output, thepositive outputs of each of the rectifier arrangements being connectedto the positive input of said first converter, the negative outputs ofeach of the rectifier arrangements being connected to the negative inputof said first converter, and the common terminal of the two sets ofstar-connected multi-phase windings being connected to the neutral ofthe electrical power generating system.
 23. An electric power supplysystem according to claim 22 wherein each of the positive and negativeoutputs of the rectifier arrangements is connected to the respectiveinput of the first converter through a respective multi-coupled windinginductance arrangement.
 24. An electric power supply system comprising:a source of AC power which is operable to provide an AC output voltagewhich has at least one phase; a series train AC/AC conversion circuitcomprising first converter means operable to establish an intermediateDC link by converting the AC output voltage into an intermediate DCvoltage and second converter means operable to convert the intermediateDC voltage into an AC output voltage having at least one phase forsupply to an external load; a bypass circuit which connects the, or eachphase of the, AC output voltage provided by said source directly to arespective phase of the AC output voltage of the second converter meansthereby bypassing the series train AC/AC conversion circuit, the bypasscircuit including an electronically operable unidirectional switcharrangement for the, or each phase of the, AC output voltage provided bysaid source, the electronically operable unidirectional switcharrangement being operable to enable current flow through the bypasscircuit as well as through the series train AC/AC conversion circuitwhen the power supply system is connected to an external load; currentsensing means operable to sense output current flow from the seriestrain AC/AC conversion circuit when the power supply system is connectedto said load; and control means responsive to said current sensing meansand operable to control said unidirectional switch arrangement andthereby to control current flow through said bypass circuit, wherein arespective trip switch is provided for the, or each phase of the, ACoutput voltage of said second converter means and the respective phaseof said bypass circuit, said unidirectional switch arrangement beingopen normally so that normally there is no current flow through saidbypass circuit but said control means being operable to close saidunidirectional switch arrangement in response to said current sensingmeans sensing current flow through said series train AC/AC conversioncircuit in excess of a predetermined current for a predetermined time sothat the flow of current from said series train AC/AC conversion circuitto said external load through said trip switch or switches is augmentedby current flow through said bypass circuit whereby the, or each, tripswitch is caused to trip by the augmented current flow through it.